Validation of a clinical prediction rule to distinguish Lyme meningitis from aseptic meningitis.

OBJECTIVES: The "Rule of 7's," a Lyme meningitis clinical prediction rule, classifies children at low risk for Lyme meningitis when each of the following 3 criteria are met: <7 days of headache, <70% cerebrospinal fluid (CSF) mononuclear cells, and absence of seventh or other cranial nerve palsy. The goal of this study was to test the performance of the Rule of 7's in a multicenter cohort of children with CSF pleocytosis.

METHODS: We performed a retrospective cohort study of children evaluated at 1 of 3 emergency departments located in Lyme disease-endemic areas with CSF pleocytosis and Lyme serology obtained. Lyme meningitis was defined using the Centers for Disease Control and Prevention criteria (either positive Lyme serology test result or an erythema migrans [EM] rash). We calculated the performance of the Rule of 7's in our overall study population and in children without physician-documented EM.

RESULTS: We identified 423 children, of whom 117 (28% [95% confidence interval (CI): 24%-32%]) had Lyme meningitis, 306 (72% [95% CI: 68%-76%]) had aseptic meningitis, and 0 (95% CI: 0%-1%) had bacterial meningitis. Of the 130 classified as low risk, 5 had Lyme meningitis (sensitivity, 112 of 117 [96% (95% CI: 90%-99%)]; specificity, 125 of 302 [41% (95% CI: 36%-47%)]). In the 390 children without EM, 3 of the 127 low-risk patients had Lyme meningitis (2% [95% CI: 0%-7%]).

CONCLUSIONS: Patients classified as low risk by using the Rule of 7's were unlikely to have Lyme meningitis and could be managed as outpatients while awaiting results of Lyme serology tests.